Vapor pressure type thermal power element



I. w. cox

VAPOR PRESSURE TYPE THERMAL POWER ELEMENT Oct.'18, 1949.

Filed Nov. 29, 1946 @xm'wv w).

Patented Oct. 18,1949

a484,: varoa raEssUaE TYPE THERMAL rowan ELEMENT Irvin W. Cox, West Allis, Wis aslignor to Cutler- Hammer, Inc., Milwaukee, Win" a corporation of Delaware Application November 29, 1948, Serial No. 713,047

Claims. 1

This invention relates to electrothermal means for the tripping of circuit breakers and for other uses, and more particularly relates to electrothermal means of the vapor pressure type.

A now widely used type of overload circuit breaker employs tripping means comprising bimetal which for fast action has passed through it the electric current relied upon for raising its temperature to tripping value under overload conditions. It is well known that a bimetal element lacks advantages of a thermal element of the vapor pressure type but heretofore there seems to have been no element of the latter type satisfactorily controllable by current traversing the same. In other words, the vapor pressure type element has required a special heating unit and this has limited its uses and as in the case of the circuit [breaker aforementioned, has resulted in preferencev being given to other types of elements although lacking certain of its highly advantageous characteristics.

An object of the present invention is to provide a vapor pressure type of thermal power element which is satisfactorily controllable by current traversing the same.

Another object is to provide such a vapor pressure element of small dimensions which will be fast in action and capable of developing adequate power for tripping circuit breakers of the aforementioned type and for other purposes.

Another object is to provide a thermal power element which may be manufactured to function properly in a circuit breaker of the type aforementioned without need of subsequent special calibration.

Another object is to provide a relatively simple and inexpensive vapor pressure element which will satisfactorily conduct its heating current and which will be rugged and durable in service.

Other objects and advantages of the invention will hereinafter appear.

One embodiment of the invention is illustrated in the accompanying drawing, it being understood that the embodiment illustrated is susceptible of various modifications without departing from the scope of the appended claims.

In the drawing,

Figure 1 shows more or less schematically a circuit breaker of the well known type aforementioned modified in respect of the tripping means for incorporation in the latter of one form of the proposed thermal power element;

Fig. 2 shows diagrammatically circuit connections for the thermal power element;

2 the power element prior to filling and sealing thereof Fig. 4 is an enlarged cross sectional view of the casing taken on line 4-4 of Fig. 3, and

Fig. 5 is an enlarged longitudinal sectional view of the power element when completed.

Referring to Fig. 1, the circuit breaker schematically shown therein is as aforeindicated of well known form except for modification of its tripping mechanism. A contact arm I fulcrumed at '2 on a slide member 3 has its lower end engaged by a latch bar 4 of rigid material with which is associated thermal power element 5 of the construction later described. When arm I is latched [by bar 4 coil spring 6 biases said arm in a direction to engage its contact 1 with a stationary contact 8', such engagement being effected when slide member 3 is released from the posi- -tion shown. As will be understood, camlever 9 provides means to move slide member 3' into the position shown and to hold it is such position subject to release at will for engagement of contacts I and 8. When the contacts I and 8 are in engagement spring 6 also subjects arm I to a bias which upon tripping of latch bar 4 causes said arm to rock on its fulcrum 2 to disengage contacts 'I and 8. As will also be understood slide member 3 when moved to the right by the cam lever will reset arm I in latched position following automatic tripping of said arm.

latch bar 4 has a fulcrum I0 on a stationary part II against which it is yieldingly held by a coil spring I2 inter-posed between said arm and a stationary part I3. Spring I2 is so positioned relative to the fulcrum of bar 4 as to bias said bar to latching position and the thermal power element is interposed between the bar 4 and fixed part II whereby upon expansion it will trip said bar against the bias of spring I2. As the power element carries current it must have insulation and while its isolation may be provided for in any desired way Fig. 1 shows the latch-bar 4 and part II as of insulating material.

As indicated in Fig. 2, the element 5 is of an elongated form and preferably it is connected, as shown in Fig. 2, in the circuit controlled by contacts 'I and 8, th connections to said element Fig. 3 is a perspective view of the casing of flattened 'form. Fig. 3 shows a preferred form.

of flattened tubing, but as will be understood assaosa 3 such form may be varied. Fig. 3 shows in full size the casing of an element successfully used in a circuit breaker of the type schematically shown in Fig. 1. Describing the casing of Fig. 3 in further detail, the opposed flattened walls of the tubing have pressed-in elongated panels 5', extending lengthwise of'the casing. Thus the form of each of the opposed walls of the casing simulates that of the bottom of an oil can and when said walls are spread and released they will spring back into a contiguous relation. This detail of the casing is best shown in Fig. 4 which as aforestated is on an enlarged scale.- Thecasing as initially formed has one or both ends leftv open to a degree suitable for filling, after which the open end or ends are suitably sealed as by welding or brazing, the finished element being without change in appearance except for sealing of its initially open ends.

End sealing of the casing preferably is effected by welding at some distance in from the end edges of the casing thus to provide electric terminal portions from which the vaporizable fill is excluded. Referring to Fig. 5 the weld preferably seals the chamber in the casing along the inner boundary lines of the relatively thin flattened end portions 5 and preferably the end portions are dipped in silver solder to better adapt them to use as electric terminals. The silver solder lodging on the portions 5 exteriorly thereof is designated 5 and preferably the solder dip is so carried out as to leave uncoated by the solder exteriorly substantial transverse sections of the end portions 5 lying between the solder and the chamber within the casing.

As will be apparent, a casing so formed will provide a relatively long current conducting path having for all cross sections of such path substantial uniformity of electrical resistance and will confine its fill to a relatively long, narrow and very shallow chamber spaced from the solder coated tips serving as electric terminals. The casing will afford emcient transfer to its fill of the heat generated in said casing by the current traversing the latter, the heat loss in the application illustrated being relatively small and mainly through conduction. Thus with a suitable fill, a sensitive and self contained thermal power element will result, and as the opposed paneled walls of the casing are both flexible the element when mounted, as shown in Fig. 1, will provide for movement of the latch bar through a range twice as wide as that of a single moving wall of the casing. Prior to production of vapor pressure within the casing the opposed paneled walls of the casing are in a contiguous relation under the influence of atmospheric pressure and the spring loading, whereas upon production of vapor pressure within the casing, as a result of a few degrees rise in temperature above a critical value, the result is a sharp reversal of force on said walls of the casing for spreading of the latter.

Water orother suitable vaporizable fill may be employed, it being understood that it is important to restrict the quantity of fill for protection of the casing against destructive internal pressures. Safe quantities of fill may be metered in various ways. One method comprises inserting into the casing before sealing a solid which may be decomposed to yield a given small quantity of vaporizable fill. Thus, for example, a length of cotton thread may be inserted into the casing and after sealing of the ends of the casing such thread may be decomposed to carbon and water by heating to a temperature of about 600 F. Unless the thermal power element is then overheated for some time at a temperature high enough to cause the water-gas reaction between the water and the residual carbon, such fill will give very satisfactory results whereas the metering is very easily accomplished with precision. A method which is applicable to a fill in liquid state such as water alone consists in taking into a capillary tube by capiliarity a given quantity of water and then by air pressure forcing the water content of such tube into the casing I, sealing the casing prior to loss of any of the water fill by evaporation or otherwise, This latter method has been found acceptably accurate but not as accurate as the former method. Also, a fill by the latter method has the advantage of being without an upper temperature limit in contrast to a fill accomplished by the former method.

Such a power element will be subjected to the aforementioned sharp expansive force at the boiling point of its liquid fill or at a few degrees rise above the boiling point. Where the element is used in the relation depicted the loss of heat will be mainly due to conduction, a loss proportional to the difference in temperature between the thermal element and the ambient, which difierence will tend to be in proportion to the square of the heating current. If the element is made to trip with certainty on a 10 rise in temperature above boiling point of the fill, which appears quite possible, it may be made to trip with certainty on a 5% increase in heating current. Thus as it appears improbable that manufacturing tolerances will be in excess of the 25% allowable in the case of the circuit breaker shown herein, it appears probable that the power element can be manufactured so as to avoid need of subsequent special calibration. The aforementioned allowance of 25% is in tripping range from rated current to rated current plus 25%, which means an overload wattage range from rated wattage in rated wattage plus 56%.

Thus where a water fill is used the tripping temperature would be about C. or about 230 F. and preferably the fill would be limited for vapor ization thereof in its entirety between 250 and 300 F. The power element would, therefore, be subiected to the maximum internal pressure at a temperature far below the red heat temperature.

A thermal power element of the character described has been found capable of tripping a circuit breaker of the herein illustrated type in less than a half cycle of the current passing through the breaker. However, if trip of the circuit breaker occurs early in the half cycle the flow of current persists even after separation of the circuit breaker contacts, until completion or near completion of the half cycle, and while the power element may be designed to carry such persistin current safely, it may be desirable to protect it against current flow after it has performed its tripping action. This can be accomplished through the medium of a shorting bar for the power element to be engaged by said element upon expansion of the latter for shunting the same. For simplicity of illustration such a bar is represented in the diagram, Fig. 2, by a normally open switch It adapted to be closed by element 5 through the medium of a part l5 when said element expands, said switch when closed shunting element 5 to reduce current flow therethrough.

It will be understood that Fig. 2 does not show the shorting bar as an essential part thereof for ordinarily it would be omitted.

Stainless steel 18.8 has been found very satisfactory for the casing of the thermal power element, but as afore-indicated other metals and alloys may be employed if desired.

Where the casing is formed as shown with marginal thickness greater than that 01' the interior expansible section it will, of course, he obviously advantageous to form the members between which it is mounted with bearing surfaces affording clearance for the thicker marginal portions and at all times bearing against the panels 5*, thereby to take advantage of the full range of spread of the panels 5*.

What I claim as new and desire to secure by Letters Patent is:

l. A vapor pressure type thermal element comprising an electric resistor of tubular form having therein vaporizable matter and being end sealed -for confinement of such matter and the vapor therefrom, said resistor which serves as an electric heater for its said content having spaced electric terminal portions, 8. flat expansible section between said terminal portions and substantial uniformity of electrical resistance for all cross sections which are between said terminal portions and transverse of current flow therebetween.

2. A vapor pressure type thermal power element comprising an end sealed tubular resistor having therein vaporizable contents and serving as an electric heater for its contents, the sealed end portions of said resistor constituting electrical terminal parts substantially solid in cross section and said resistor having between said end portions an expansiblefiat portion and having for all its cross sections transverse oi the current fiow between said end portions substantial uniformity of electrical resistance.

3. A vapor pressure type thermal power element comprising a seamless and end sealed tubular resistor element having therein vaporizable contents and sewing as an electric heater for its contents, said element havin spaced terminal parts and having intermediate thereof an expansible flat section and said element for all cross sections thereof between said terminal parts and transverse of the current fiow having substantial uniformity of electrical resistance.

4. A vapor pressure type thermal power ele- A ment comprising volatile matter and as a casing and as an electrical heater for said volatile matter a metal tube with electrical resistance properties having its ends sealed and having medially of its ends a flattened section with normally contiguous yielding walls to be spread by internal pressure.

5. A vapor pressure type thermal Power element comprising volatile matter and as a casin and as an electric heater for said volatile matter a metal tube having electrical resistance properties and its ends sealed and having medially of its ends a flattened section expansible "by internal pressure. the opposed walls of said flattened section having resiliency and being self-biased toward one another.

6. A vapor pressure type thermal power element comprising volatile matter as a casing and as an electric heater for said volatile matter a metal tube with electrical resistance properties having its opposite ends sealed and having medially thereof a flattened section expansible by internal pressure, the opposed walls of said fiattened section having pressed-in panels which are resilient and self-biased to a contiguous relation.

'7. A vapor pressure type thermal power element comprising a metal tubular casing having electrical resistance properties and a vaporizab'le fill therefor, said casing being sealed at its ends and having therebetween a section expansible in a direction transverse to the longitudinal axis of said casing by internal pressure, and said casing when traversed by an electric current constituting an electric heater for its fill, said fill when vaporized eifecting spread of opposed walls of said expansible section of said casing.

8. A vapor pressure type thermal power element comprising a metal tubular casing having electrical resistance properties and a vaporizable fill therefor, said casing being sealed at its ends and having an intermediate fiat portion, and said casing when traversed b an electric current constituting an electric heater for its fill, said fill when vaporized efiecting spread of opposed walls of said flat section of said casing, and said walls having resiliency and tending to return to a contiguous relation.

9. The combination with a vapor pressure type thermal power element comprising a tubular metal casing having electrical resistance properties and having intermediate its ends an expansible fiat section; and a vaporizable fill therefor, said casing being sealed at opposite ends and when traversed by an electric current constituting a heater for its fill, which fill when vaporized eilects spread of opposed walls of said expansible section, of a mounting for said power element comprising relatively movable members between which said power element is confined with its flat walls in engagement with said members.

10. The combination with a vapor pressure type thermal power element comprising a tubular metal casing having electrical resistance properties and having intermediate its ends an expansible flat section, and a vaporiza-ble fill therefor, said casing being sealed at opposite ends and when traversed by an electric current constituting a heater for its fill, which fill when vaporized efiects spread of opposed fiat walls of said expansible section, oi a mounting for said power element comprising relatively movable members between which said power element is confined with its fiat walls in engagement with said members, said mounting further comprisin spring means acting through said members to load said power element.

IRVIN W. COX.

REFERENCES CITED The following references are of record in the die of this patent:

UNITED STATES PATENTS Number Name Date 1,547,632 Wensley July 28, 1925 1,612,246 Whittingham Dec. 28, 1926 1,678,372 Whittingham July 24, 1928 1,836,634 Urfer Dec. 15, 1931 2,022,907 Worley Dec. 3, 1935 2,247,414 Stewart July 1, 1941 2,359,676 Raney Oct. 3. 1944 

